Chemical toxicity of thorium in Saccharomyces cerevisiae

Thorium(Th),an actinoide element, is among the most common naturally-occurring radioactive materials found in the environment. Thorium was used as a radiographic contrast agent(thorotrast)from 1930 to 1955, and many studies on its effects on the human body have been reported. Once thorium is absorbed, the risk of cancer increases due to direct bombardment from alpha-particles with high linear energy transfer during thorium decay. Most of these reports focused on irradiation damage caused by long-term thorium exposure. In contrast, acute thorium toxicity derives more from thorium’s chemical reactivity than its radioactive decay. Here, we report the chemical toxicity of thorium using yeast DNA microarray analysis. Thorium treatment of yeast was found to induce 136 genes. Genes involved in“C-compound and carbohydrate metabolism”,“energy”,“cell rescue, defense and virulence”and“biogenesis of cellular component”were induced significantly by thorium treatment. These genes were further classified into the following processes: oxidative stress(7 genes),glycogen and trehalose metabolism(6 genes),and cell wall damage(10 genes).However, only one gene related to DNA damage was induced. These results indicate that thorium causes cell wall damage and induces oxidative stress, and suggest that in order to overcome oxidative stress, cells promote metabolism of energy reserves in the form of glycogen and trehalose.